1. Technical Field
The present invention relates to a method of and apparatus for evaluating the position of a disturbance on a transmission link, in particular where the disturbance is a time-varying disturbance.
2. Related Art
It is known to estimate the position of an irregularity in a waveguide by launching a test pulse in the waveguide, and monitoring the distributed backscattering of the test pulse so as to detect an abnormality, if any, in the time-dependence of the returned signal. An example of an abnormality could be a step change in the time-dependent amplitude of a return signal. However, such known Optical Time Domain Reflectometry (OTDR) techniques are not well suited to detecting time-varying disturbances.
According to the present exemplary embodiment, there is provided a method of evaluating the position of a time-varying disturbance on a transmission link, the method including the steps of: copying, at least in part, an output signal from a source, such that there is a pair of signal copies; transmitting the signal copies onto the transmission link; receiving from the transmission link at least partially returned signal copies previously transmitted thereon; combining the received signal copies of a transmitted pair so as to produce a combination signal; and, using a temporal characteristic in the combination signal to evaluate the position of the disturbance on the communications link.
Because copies of the output signal are combined to produce a combination signal, any modification of at least one of these copies is likely to produce a change in the combination signal, thereby facilitating the evaluation of a time-varying disturbance.
Preferably, the transmission link will be an optical waveguide, such as an optical fibre. A dynamic or other time-varying disturbance of the link will normally be a physical disturbance, such as an acoustic or other vibration. A physical disturbance is likely to produced a strain or an elastic wave in the optical medium of the transmission link, thereby changing the phase of at least one of the signal copies travelling along the link. A dynamic disturbance may be stationary, that is, located at a stationary point. Alternatively, the dynamic disturbance may move along the optical waveguide.
Preferably, a signal will be returned by a process of distributed backscattering, for example Rayleigh backscattering, such that the signal is returned progressively as the signal propagates along the waveguide. This will normally give rise to a combination signal that is distributed over time. The combination signal will preferably be monitored to detect a disturbance feature therein, from which disturbance feature the presence of a disturbance may be inferred.
One example of a disturbance feature may be a change in the amplitude of the combination signal, for example a step change with respect to time. The combination signal may be displayed on a display devices as a function of time in the manner of a trace, allowing the occurrence of a disturbance to be visually inferred from the trace. However, the combination signal may be analysed to detect more subtle characteristics of a disturbance feature.
A temporal characteristic in the combination signal may be a return time associated with the disturbance feature, in particular the arrival time of returned signal copies (once combined) responsible for a disturbance feature, this time being the round-trip time for light propagating to and from the position of the physical disturbance. In one embodiment, a pulse form the source will give rise to a time distributed combination signal, and the combination signal will be monitored as a function of the elapsed time from a reference time, which reference time will be related to the time at which the pulse is generated.
The output signals from a source will preferably have an irregular component, in which case the step of copying at least in part the output signal from the source will preferably result in that the irregular component is common to each of the signal copies of a pair. Other characteristics of the signal need not be the same in each signal copy: for example, the signal copies may have different amplitudes. The irregular component will preferably be random, or pseudo random (by pseudo random; it is meant that although in theory a component is possible to predict, the time or processing power required to do this will make it in practice impossible to predict). If the output signal has a waveform, the irregular component may be provided by the phase of the waveform if the waveform has randomly occurring phase changes. The waveform may conveniently be provided by an optical source having a short coherence time, preferably less than 10 pico seconds or even less than 1 pico second. The combination signal will preferably be an interference signal resulting from the interference or mixing of two waveforms.
Preferably, the signal copies will be transmitted along the transmissions link with a time delay relative to one another, such that there is a leading copy and a trailing copy. The returned leading copy can then be delayed relative to the previously trailing copy, such that both copies can be combined substantially in step with one another.
In a preferred embodiment, this is achieved using an interferometer stage, such as an unbalanced Mach Zehnder interferometer. In this preferred embodiment, the output from the optical source is fed to the interferometer, where the signal is copied, one copy being channelled to one path of the interferometer, the transit time associated with each path being different, such that a relative or differential delay results between the time at which the signal copies are transmitted from the interferometer stage. The same interferometer stage can then be employed to re-align the returned signal copies in a particularly convenient manner, since the relative delay imposed in the outbound direction will be the same as the relative delay imposed in the return direction, this being in each case determined by the difference in the transit times of the two paths.
The differential delay will preferably be chosen in dependence at least in part on the average coherence time of the source. The differential delay will preferably be much longer than the coherence time. Preferably, the ratio of the differential delay to the coherence time will be greater or equal to 103 yet more preferably 105 or even yet more preferably 107.
The steps of copying output signals and transmitting the signals will preferably be carried out at a first location, a disturbance remaining detectable at distance of at least 1 km or even at least 10 km from the first location.
The transmission link may include an optical channel extending along a guide track, the guide track being arranged to guide the movement of a moving vehicle. In this way, the transmission link may be used to monitor the movement of a vehicle guided to move along the transmission link.
Preferably, the optical channel will be in a mechanically coupled relationship to the track, such that movement of the vehicle causes a disturbance along the optical channel. The optical channel need not be directly coupled to the track, and may be indirectly coupled for example through the ground. For example, the optical channel may simply lie on the ground next to the track or in a duct, without necessarily being secured. However, because of the mechanical coupling, vibrations or other disturbances such as acoustic disturbances generated by the movement of the vehicle along the track may cause a corresponding (albeit attenuated or modified) disturbance in the transmissions link, which disturbance will move along the link in a continuous fashion at the same speed as the vehicle.
Because the acoustic coupling to the optical channel need not (but may) be enhanced at specific coupling points, and because instead the continuous coupling provided by the medium between the track and the channel will normally be sufficient, an optical channel provided by an existing track-side fibre cable may conveniently be used. Alternatively, a new cabled installed in an existing track-side duct may be employed.
The guide track will preferably have the form of one or more rails, which rails will normally guide the movement of a train. This arrangement is particularly convenient because the vibrations generated by train are likely to be significant, thereby making the position of such a vehicle easier to detect.